Radio operators are assigned a handle or call sign to uniquely identify themselves. For amateur radio operators, this is typically a series of letters and numbers (pronounced phonically - see below). For the military, identification usually changes every day, or even multiple times a day for security, but usually utilizes the vehicle number where the radio is located as part of the identification. For example, it comes down from the chain of command that the new prefix is Sapper. So if your vehicle number is 7, you would be "Sapper 7". A favorite joke among members of the military taking advantage of this fact is:

Have you ever had to spell something over the phone? Then you probably know how annoying it can be to distinguish between various letters such as B, D, P, etc. Squelch, static, and weak signals on a radio net complicate things even more. It is for this reason that you never just say a letter in radio communication - you use its phonic equivalent.

Numbers are given similar treatment to letters. So as to avoid confusion, large numbers are "spelled out" instead of spoken normally. For example, instead of saying 1593 as "one thousand five hundred ninety three", it would be pronounced "one fife niner tree"